1. Field of the Invention
The present invention relates to a digital broadcast receiver for receiving digital broadcast using OFDM, and particularly relates to a channel search method for digital terrestrial broadcasting.
2. Description of the Related Art
An Orthogonal Frequency Division Multiplexing (OFDM) has been proposed as a system for transmitting a digital signal in recent years. In the OFDM system, data is transmitted by making use of a plurality of carriers orthogonal to each other on the frequency domain. As a result, an OFDM transmitter modulates a transmission signal by utilizing Inverse Fast Fourier Transformation (IFFT), and an OFDM receiver demodulates the transmission signal by using Fast Fourier Transformation (FFT). Since OFDM is highly efficient in frequency usage, its application to digital terrestrial broadcasting has been widely explored. It should be noted that OFDM has been employed in ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), which is a standard of digital terrestrial broadcasting in Japan.
Digital TV broadcasting employing the UHF band (channels 13-62) and digital radio broadcasting employing the VHF band (channels 7-8) are used for digital terrestrial broadcasting (ISDB-T) in Japan. In digital TV broadcasts, as shown in FIG. 1A, a 6 MHz band is assigned to each channel and the band is divided into 13 segments. High-definition broadcasts with 12 segments are sent to common television sets (fixed terminals), and broadcasts with the remaining one segment are sent to mobile terminals. Additionally, there are some channels that transmit broadcasts with 13 segments to fixed terminals. Note that in digital radio broadcasts, as shown in FIG. 1B, a 6 MHz band is divided into eight segments and broadcasts are sent using either one segment or three segments.
At present, only some of the above 50 channels (52 channels when digital radio is included) are actually used for digital broadcasting. Therefore, the digital broadcast receiver should comprise a function to search for channels being used for the digital broadcasting (or a function to search for available channels). In the following description, such a function is referred to as a channel search function.
It should be noted that when receiving digital terrestrial broadcasts with a mobile terminal, since the reception environment changes from hour to hour in accordance with the location of the mobile terminal, the terminal might not always be able to receive radio waves of the channel in which the digital broadcast is being transmitted. For that reason, the channel search function is particularly important in mobile terminals.
FIG. 2 is a flowchart showing an example of a channel search procedure in the related art. Note that processing in the flowchart is performed for each channel.
In step S101, whether the frame synchronization has been established or not is checked. The frame synchronization is detected by employing a synchronization signal in TMCC (Transmission and Multiplexing Configuration Control). When the frame synchronization is established, a bit error ratio (BER) is detected in step S102. If the bit error ratio is less than a prescribed threshold, it is determined that “the corresponding channel is being broadcast (or is receivable)”. Meanwhile, when the frame synchronization is not established or the bit error ratio exceeds the threshold, it is determined that “the corresponding channel is not being broadcast (or is not receivable)”. Note that a method employing the result of error correction sequence instead of detecting the bit error ratio is also proposed.
In Patent Document 1 (Japanese Patent Application Publication No. 2005-333190), a technology for obtaining program sequence information and for reducing the channel search time using the information is described. Patent Document 2 (Japanese Patent Application Publication No. 2003-110536), although it is not directly related to the channel search, describes a technology that speeds up the switching of broadcasts in terminals that can receive 1-segment broadcasts and 3-segment broadcasts.
The channel search procedure shown in FIG. 2 requires a time of approximately 0.7-1.0 second from the start of OFDM operation to the end of the frame synchronization check, and also requires approximately 0.5 second for monitoring the bit error ratio. If the procedure shown in FIG. 2 is performed on all channels in the UHF band (i.e. 50 channels), 1-2 minutes are required for a channel search. For that reason, users may feel frustrated or inconvenienced when conducting a channel search.
It should be noted that there is another possible method for reducing the channel search time: omit step S102. However, if the channel determination is performed solely on the basis of whether or not the frame synchronization has been established, statistically wrong determination results may be obtained (mainly the determination of “the corresponding channel is being broadcast” even though the corresponding channel is not being broadcast).